1. Field of the Invention
The present invention relates to a hot melt ink utilizable for an ink jet printer, in particular to a hot melt ink which exists in a solid state at a room temperature (ordinary temperature) and is used in an ink jet printer after being thermally melted over the room temperature when printing is conducted by the ink jet printer.
2. Description of Related Art
In general, an ink jet printer has various advantages in printing because of being noiseless, high speed printing, high quality printing and ability of color printing. On the other hand, since the ink jet printer conducts printing by directly adhering onto a print sheet ink droplets jetted from nozzles, printing quality or drying time of the ink droplets is influenced by quality of the print sheet, as a result, there is a problem that printing quality of the ink jet printer remarkably goes down when printing is conducted onto a print sheet with low quality.
In order to dissolve the above problem, it has been conventionally proposed an ink jet printer which can print images with ink dots having uniform diameters onto the print sheet in spite of quality of the print sheet, by using a hot melt type ink which has a melting point higher than the room temperature. As ink composition of the hot melt ink, it is known an ink including a natural wax disclosed in the specification of U.S. Pat. No. 4,390,369 (Japanese Patent Application Laid Open No. Sho 58-108,271), an ink including stearic acid disclosed in the specification of U.S. Pat. No. 4,758,276 (Japanese Patent Application Laid Open No. SHo 59-22,973), and an ink which includes acid or alcohol of carbon number 20-24 and acid or alcohol of carbon number 14-19 and includes ketone having a relatively high melting point and a dye as colorant, disclosed in the specification of U.S. Pat. No. 4,659,383 (Japanese Patent Application Laid Open No. Sho 61-83,268). Further, it is disclosed in U.S. Pat. No. 4,659,383 and Japanese Patent Application after substantive examination, Laid Open No. Hei 2-18,710, a hot melt ink in which a solid pigment is dispersed in wax having a melting point higher than 65.degree. C., or fatty acid or alcohol of carbon number 18-24.
In Japanese Patent Application Laid Open No. Hei 5-125,316, it is disclosed a hot melt ink composition which includes a natural wax having penetration grade less than 2, the penetration grade being measured by a penetration testing apparatus and appreciated based on a depth that a needle penetrates under a predetermined condition according to Japanese Industrial Standard (JIS) K2207, K2220, K2235, a dye dissolving agent with a melting point higher than 80.degree. C. and a dye. By utilizing such hot melt ink, it can prevent inferiority of printing quality occurring due to blurring generated when the ink is re-melted during ink fixation by heat or pressure, or due to offset generated when the ink is adhered to a pressure roller.
Further, it is disclosed in Japanese Laid Open Nos. Hei 2-167,373, Hei 2-206,661 and Hei 5-194,899, a hot melt ink in which an antioxidant is added thereto.
However, in the hot melt ink using a pigment as a colorant among the conventional hot melt inks mentioned above, the pigment itself has a heat resistant stability, but does not have a sufficient dispersing stability. Accordingly, it is apt to occur blinding of orifices in an ink jet head installed in the ink jet printer or deviation of flying curves of ink droplets ejected from the orifices. Further, such hot melt ink including pigment cannot realize color clearness obtained by the hot melt ink including a dye as a colorant.
On the contrary, in case of the hot melt ink using a dye as a colorant, there scarcely occurs blinding of orifices or deviation of flying curves of ink droplets since the dye itself sufficiently dissolves in the ink. Thus, clear printing can be conducted by combining the dye and a suitable dye dissolving agent (color coupler). However, while conserving the hot melt ink in a melted state by heating for a long time, the dye itself or the dye dissolving color coupler is thermally deteriorated. As a result, there will occur a problem that discoloration and deposition of the dye are generated.
In particular, in case that a xanthene dye is used as the dye in the hot melt ink, coloring thereof is obstructed due to thermal deterioration of the dye dissolving color coupler, thereby there is a heavy problem that impermissible discoloration occurs. However, it is a fact that the xanthene dye as a colorant can realize an excellent clearness of magenta color, which can never be obtained by the other dye, among three primary colors in printing such as cyan color, magenta color and yellow color, by combining a suitable dye dissolving color coupler.
As mentioned above, the xanthene dye has different two aspects: the first aspect is that the xanthene dye is very useful for a colorant in the hot melt ink; the second aspect is that the xanthene dye is apt to be discolored by heating.
Under the above situation, in the hot melt ink disclosed in Japanese Laid Open Nos. Hei 2-167,373, Hei 2-206,661 and Hei 5-194,899 (previously raised), the ink is improved by using the antioxidant therein so that viscosity of the ink is not increased when heated and the ink concludes to have light resistance. However, it is not considered a problem of heat discoloration which occurs in case that the xanthene dye is used as a colorant in the hot melt ink.